1. Field of the Invention
The present invention relates to a process for coating medical materials to be used in vivo, such as bone repair materials, intravitally embedding medical instruments or accessories, medical implements, various artificial organs or the like, more particularly, to a process for forming a bioactive hydroxyapatite film having bone-like structure and composition over surfaces of any materials to be used in vivo, inorganic materials, metallic materials or organic materials.
2. Description of the Prior Art
As a process for coating a substrate with hydroxyapatite, there have been generally known plasma spray coating processes (Japanese Patent Application Laid-open Nos. 62-34,559, 62-57,548, and 63-160,663) and processes wherein a substrate is coated with a solution or compound containing Ca and P, followed by sintering (Japanese Patent Application Laid open Nos. 62-231,669, 63-24,952 and 63-46,165). Other than the above, there have been disclosed a sputtering process (Japanese Patent Application Laid-open No. 58-109,049) and, recently, a flame spraying process (Proceedings of Fall Meeting, The Ceramic Society of Japan, 1988, p.p. 401.about.402), a glass frit firing process (Abstract of 9th Annual Meeting of Japanese Society for Biomaterials, 1987, p.6) and an electrophoretic coating process (Proceedings of Fall Meeting, The Ceramic Society of Japan, 1988, p.p. 417.about.418).
Each of the above-described prior art processes has disadvantages such as the following or the like:
1 in the plasma spray coating process and the flame spraying method, intricate and expensive pieces of equipment are required, dense films are difficult to produce, and the raw material, i.e., hydroxyapatite, is once molten at a high temperature so that the resulting apatite films are different in type from the bone apatite; PA0 2 in the sputtering process, also intricate and expensive pieces of equipment are required, and the raw material, hydroxyapatite, is once decomposed due to high energy so that the resulting films are different in type from the bone apatite; PA0 3 in the sintering process or the glass frit firing process, since a heat treatment at around 850.degree. C. is required, only substrates high in heat resistance are applicable and, further, also in this case, the raw material, i.e., hydroxyapatite, is once subjected to a heat treatment at a high temperature so that the resulting apatite films are different in type from the bone apatite; and PA0 4 in the electrophoretic coating process, since substrates themselves are used as an electrode, only good electro-conductive, metallic substrates are applicable, and a sintered apatite is used as a raw material so that the resulting apatite films are also different from the bone apatite.
In the meanwhile, we, the inventors, have already developed a glass-ceramic, by a process for separating out apatite and wollastonite within a glass, which exhibits a bioactivity to spontaneously bond chemically and strongly with bones in vivo within a short period of time and retains a high mechanical strength for a long period of time. In the further pursuit of factors controlling this bioactivity, the inventors have elucidated that what plays an important role when ceramics are bonded with bones is not a apatite phase existing in the glass-ceramics that is formed by glass crystallizing process but a bone-like apatite layer newly depositing on the surface of the glass-ceramics by reacting with ambient body fluid when they are embedded in the living body. Further, it has been found that this apatite layer can be produced only by soaking the glass-ceramics to be bonded with the bones in an aqueous solution not containing cells but having only its inorganic ion concentration made to be equivalent to the human body fluid, and that Ca and Si eluting from the glass-ceramic play a very important role for producing this apatite layer. Based on these findings, the inventors have conducted assiduous studies to form a bone-like apatite layer on the surface of various substrates and eventually accomplished the present invention.